NEC & CEC Explosive Location Class & Division Definitions

Class I Locations are those in which flammable gases or vapors are, or may be, present in the air in quantities sufficient to produce explosive or ignitable mixtures.

Class I, Division 1 Location

Where hazardous concentrations of flammable gases or vapors exist continuously, intermittently, or periodically under normal conditions,

Where hazardous concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage, or

Where breakdown or faulty operation of equipment or processes which might release hazardous concentrations of flammable gases or vapors, might also cause Simultaneous failure of electrical equipment.

Class I, Division 2 Location

Where flammable volatile liquids or flammable gases are handled, processed or used, but in which the hazardous liquids, vapors, or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems, or in case of abnormal operation of equipment,

Where hazardous concentrations of gases or vapors are normally prevented by Positive mechanical ventilation, but which might become hazardous through failure or abnormal operation of the ventilating equipment, or

Which are adjacent to Class I, Division 1 locations, and to which hazardous concentrations of gases or vapors might occasionally be communicated unless such communication is prevented by adequate positive pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.

Class II Locations are those which are hazardous due to the presence of Combustible dust.

Class III Locations are those which are hazardous due to the presence of easily ignitable fibers or flyings, but in which such fibers or flyings are not likely to be in suspension in air in quantities sufficient to produce ignitable mixtures.

Note : It cannot be assumed that equipment suitable for Class I locations is therefore suitable for Class II and III, or that equipment suitable for Class II locations is similarly automatically suitable for Class III locations.

Equipment intended for use in Class I, Division 1 Hazardous Locations
Equipment intended for use in Class I, Division 1 hazardous locations is usually either explosion-proof, intrinsically safe, or purged / pressurized.

Definitions :

Explosion-proof apparatus: "Apparatus enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby." Refer NFPA 70.

Intrinsically safe apparatus: "Apparatus in which all the circuits are intrinsically safe." Refer UL 913.

Intrinsically safe circuit: "A circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air under prescribed test conditions." Refer UL 913.

Purging: "The process of supplying an enclosure with a protective gas at a sufficient flow and positive pressure to reduce the concentration of any flammable gas or vapor initially present to an acceptable level." Refer NFPA 496.

Pressurization :"The process of supplying an enclosure with a protective gas with or without continuous flow at sufficient pressure to prevent the entrance of a flammable gas or vapor, a combustible dust, or an ignitable fiber." Refer NFPA 496.

Equipment intended for use in a Class I, Division 2 Hazardous Locations

Equipment intended for use in Class I, Division 2 area is usually of the non-incendive, non-sparking, purged/pressurized, hermetically sealed, or sealed device type.

Non-incendive circuit: "A circuit in which any arc or thermal effect produced under intended operating conditions of the equipment is not capable, under the test conditions specified, of igniting the specified flammable gas- or vapor- air mixture." Refer UL 1604.

Non-incendive component: "A component having contacts for making or breaking an incendive circuit and the contacting mechanism shall be constructed so that the component is incapable of igniting the specified flammable gas- or vapor-air mixture. The housing of a non-incendive component is not intended to: exclude the flammable atmosphere, or contain an explosion." Refer UL 1604.

Non-incendive field circuit: "A circuit that enters or leaves the equipment enclosure and that under intended operating conditions is not capable, under the test conditions specified, of igniting the specified flammable gas- or vapor-air mixture or combustible dust." Refer UL 1604.

Purging: "The process of supplying an enclosure with a protective gas at a sufficient flow and positive pressure to reduce the concentration of any flammable gas or vapor initially present to an acceptable level." Refer NFPA 496.

Pressurization :"The process of supplying an enclosure with a protective gas with or without continuous flow at sufficient pressure to prevent the entrance of a flammable gas or vapor, a combustible dust, or an ignitable fiber." Refer NFPA 496.

Hermetically sealed component: "A component that is sealed against entrance of an external atmosphere and in which the seal is made by fusion, such as soldering, brazing, welding, or the fusion of glass to metal." Refer UL 1604.

Sealed device: "A device that is constructed so that it cannot be opened, has no external operating mechanisms, and is sealed to restrict entry of an external atmosphere without relying on gaskets. The device may contain arcing parts or internal hot surfaces." Refer UL 1604.

Equipment intended for use in Class II, Division 1 area is usually of the dust-ignition-proof, intrinsically safe, or pressurized type. Dust-ignition-proof : "Enclosed in a manner that will exclude dusts and, where installed and protected in accordance with the NEC, will not permit arcs, sparks or heat otherwise generated or liberated inside of the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specified dust on or in the vicinity of the enclosure." Refer NFPA 70.

Intrinsically safe apparatus: "Apparatus in which all the circuits are intrinsically safe." Refer UL 913.

Intrinsically safe circuit: "A circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air under prescribed test conditions." Refer UL 913.

Pressurization :"The process of supplying an enclosure with a protective gas with or without continuous flow at sufficient pressure to prevent the entrance of a flammable gas or vapor, a combustible dust, or an ignitable fiber." Refer NFPA 496.

Equipment intended for use in Class II, Division 2 area is usually of the dust-tight, non-incendive, non-sparking, or pressurized types.

Dust-tight: "Constructed so that dust will not enter the enclosing case under specified test conditions. An example of such conditions would be a UL Type 12 enclosure." Refer NFPA 70.

Non-incendive circuit: "A circuit in which any arc or thermal effect produced under intended operating conditions of the equipment is not capable, under the test conditions specified, of igniting the specified flammable gas- or vapor- air mixture." Refer UL 1604.

Non-incendive component: "A component having contacts for making or breaking an incendive circuit and the contacting mechanism shall be constructed so that the component is incapable of igniting the specified flammable gas- or vapor-air mixture.

The housing of a non-incendive component is not intended to: exclude the flammable atmosphere, or contain an explosion." Refer UL 1604.

Non-incendive field circuit: "A circuit that enters or leaves the equipment enclosure and that under intended operating conditions is not capable, under the test conditions specified, of igniting the specified flammable gas- or vapor-air mixture or combustible dust." Refer UL 1604.

Pressurization :"The process of supplying an enclosure with a protective gas with or without continuous flow at sufficient pressure to prevent the entrance of a flammable gas or vapor, a combustible dust, or an ignitable fiber." Refer NFPA 496.

Equipment intended for use in Class III, Division 1 area is usually of the dust-tight or intrinsically safe type.

Dust-tight: "Constructed so that dust will not enter the enclosing case under specified test conditions. An example of such conditions would be a UL Type 12 enclosure." Refer NFPA 70.

Intrinsically safe apparatus: "Apparatus in which all the circuits are intrinsically safe." Refer UL 913.

Intrinsically safe circuit: "A circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air under prescribed test conditions." Refer UL 913.

Equipment intended for use in Class III, Division 1 area is usually of the dust-tight or intrinsically safe type.

Dust-tight: "Constructed so that dust will not enter the enclosing case under specified test conditions. An example of such conditions would be a UL Type 12 enclosure." Refer NFPA 70.

Intrinsically safe apparatus: "Apparatus in which all the circuits are intrinsically safe." Refer UL 913

NEC & CEC Hazardous Location Class & Zone Definitions

Class I, Zone 0 Locations

A Class I, Zone 0 location is a location where ignitable concentrations of flammable gases, vapors or liquids:

are present continuously; or
are present for long periods of time.

Class I, Zone 1 Locations

A Class I, Zone 1 location is a location where ignitable concentrations of flammable gases, vapors or liquids:

are likely to exist under normal operating conditions;
may exist frequently because of repair or maintenance operations or leakage; or
may exist because of equipment breakdown that simultaneously causes the equipment to become a source of ignition; or are adjacent to a Class I, Zone 0 location from which ignitable concentrations could be communicated.

Class I, Zone 2 Locations

A Class I, Zone 2 location is a location:
where ignitable concentrations of flammable gases, vapors or liquids are not likely to occur in normal operation or, if they do occur, will exist only for a short period;
where volatile flammable liquids, or flammable gases or vapors exist, but are normally confined within closed containers
where ignitable concentrations of gases, vapors, or liquids are normally prevented by positive mechanical ventilation;
adjacent to a Class I, Zone 1 location from which ignitable concentrations could be communicated.

Equipment intended for use in a Class I, Zone 0 Hazardous Locations

Equipment intended for use in a Class I, Zone 0 area is usually of the intrinsically safe, "ia," type.

Intrinsically safe apparatus: "Apparatus in which all the circuits are intrinsically safe." Refer UL 913.

Equipment intended for use in a Class I, Zone 1 Hazardous Locations

Equipment intended for use in a Class I, Zone 1 area is usually of the flameproof, purged / pressurized, oil immersed, increased safety, encapsulated or powder filled type.

Flameproof: "The enclosure of the equipment will withstand an internal explosion, and prevent passage of flame to the surrounding atmosphere. Care must be taken to maintain the length and clearance (gap) of flameproof joints in service." Refer UL 2279.

Purging: "The process of supplying an enclosure with a protective gas at a sufficient flow and positive pressure to reduce the concentration of any flammable gas or vapor initially present to an acceptable level." Refer NFPA 496.

Pressurization :"The process of supplying an enclosure with a protective gas with or without continuous flow at sufficient pressure to prevent the entrance of a flammable gas or vapor, a combustible dust, or an ignitable fiber." Refer NFPA 496.

Increased safety: "The equipment contains no normally arcing parts, and additional measures (such as increased spacing of wiring connections) are taken to prevent the possibility of high temperatures or sparks. A minimum Ingress Protection rating of IP
54 is required." Refer UL 2279.

Equipment intended for use in a Class I, Zone 2 Hazardous Locations

Equipment that is intended for use in a Class I, Zone 2 area is usually of the non-incendive, non-sparking, restricted breathing, hermetically sealed or sealed device type.

Non-incendive circuit: "A circuit in which any arc or thermal effect produced under intended operating conditions of the equipment is not capable, under the test conditions specified, of igniting the specified flammable gas- or vapor- air mixture." Refer UL 1604.

Non-incendive component: "A component having contacts for making or breaking an incendive circuit and the contacting mechanism shall be constructed so that the component is incapable of igniting the specified flammable gas- or vapor-air mixture. The housing of a non-incendive component is not intended to: exclude the flammable atmosphere, or contain an explosion." Refer UL 1604.

Non-incendive field circuit: "A circuit that enters or leaves the equipment enclosure and that under intended operating conditions is not capable, under the test conditions specified, of igniting the specified flammable gas- or vapor-air mixture or combustible dust." Refer UL 1604.

Restricted breathing: "The enclosure relies on tight seals and gaskets to prevent diffusion of the explosive atmosphere into the equipment enclosure. Provision for checking that the restricted breathing properties of the enclosure are maintained is provided." Refer UL 2279.

Hermetically sealed component: "A component that is sealed against entrance of an external atmosphere and in which the seal is made by fusion, such as soldering, brazing, welding, or the fusion of glass to metal." Refer UL 1604.

Sealed device: "A device that is constructed so that it cannot be opened and is sealed to restrict entry of an external atmosphere. The device may contain arcing parts or internal hot surfaces." Refer UL 2279.